About mid nineties a "balance shaft" was added to the 4.3. V-6 engines tend to be unbalanced "shakers". These wern't bad, but GM wanted a smoother engine. They imitated what a few foreign builders were doing and added the balance shaft. It's function is to shake the engine internally in the opposite direction of the engines natural harmonics. Thus canceling the vibrations out at certain rpms. Usually at cruising speed.